Electrophotography is recently used and applied extensively not only in the field of copiers but also in the field of various printers, because of the advantages thereof such as instantaneousness, the ability of produce high-quality images, etc. With respect to photoreceptors, which are the core of electrophotography, photoreceptors employing, as the photoconductive material thereof, an organic photoconductive material having advantages such as non-pollution, ease of film formation, and ease of production are being developed in recent years.
There recently is a trend from monochrome toward full-color in both the copiers and printers which utilize electrophotography. Modes of the formation of a full-color image mainly include the tandem mode and the four-cycle mode. Meanwhile, modes of transfer to printing media include the direct transfer mode, transfer drum mode, intermediate transfer mode, multiple development/en bloc transfer mode, etc. Unlike apparatus for forming monochromatic images, apparatus for forming full-color images are required to satisfy an exceedingly high image quality level. Under these circumstances, there is a desire for a photoreceptor which can maintain high image quality even when suffering excess stress in special environments (e.g., high-temperature and high-humidity environment).
With respect to the quality of images produced with photoreceptors, the charge transport substances greatly contribute thereto. Various charge transport substances have been put to practical use, such as carbazole derivatives, hydrazone derivatives, stilbene derivatives, butadiene derivatives, and enamine derivatives. For example, it has been proposed that a triarylamine/stilbene hybrid compound having high sensitivity be used as a charge transport substance for electrophotographic photoreceptors (patent document 1).